Stable distillate fuels with low sulfur contents and high cetane numbers are desired because of their low emissions and good engine performance. Fuels of this type can be prepared from Fischer-Tropsch products. The preparation of distillate fuels from Fischer-Tropsch processes is well known.
The Fisher Tropsch process is typically divided into two types—high temperature and low temperature. The high temperature process produces primarily light gaseous products between methane and about C8. The products from high temperature processes are highly olefinic, and distillate fuels are produced by oligomerizing the olefins. The low temperature process produces a heavier product from methane to a material having more than 100 carbon atoms. Depending on the catalyst and process conditions, the product composition can vary from highly paraffinic to a mixture of paraffins, olefins, and oxygenates. The oxygenates are primarily alcohols, but acids may also be present along with smaller quantities of other oxygenates. The structures of the products are primarily linear, e.g. normal paraffins, primarly linear alcohols and fatty acids. When the Fischer-Tropsch product comprises components other than the paraffins, the Fischer-Tropsch product may exhibit problems with stability.
Fischer-Tropsch products from low temperature processes are typically converted into distillate fuels by hydroprocessing operations which saturate the olefins and convert all the oxygenates into paraffins. These processes require the use of expensive hydrogen and expensive high pressure facilities and recycle compressors. It would be preferable not to hydroprocess all of the Fischer-Tropsch products, especially those that are already in the distillate boiling range.
One method to avoid hydroprocessing all of the Fischer-Tropsch products is to simply send the lighter fractions around the hydroprocessing unit and blend them directly into the distillate product without further treatment. The heavier fractions are converted into additional distillate product by hydrocracking. The distillate product from the hydrocracker and the lighter fractions directly from the Fischer-Tropsch process are blended. This type of operation, and the preparation of distillate fuel containing olefins, has been described several times in the literature:
By way of example, “Upgrading of Light Fischer-Tropsch Products, Final Report”, by P. P. Shah, Nov. 20, 1990 describes work performed under Contract No. AC22-86PC90014. DE91011315 (DOE/PC/90014-TB). FIG. 4.1 on page 4.14 of the report shows a Fischer-Tropsch product from an Arge reactor being separated into a C12–C18 fraction and a C19+ fraction. The C19+ fraction is hydrocracked to form additional C12-18 products, and the raw C12-18 fraction from the Fischer-Tropsch unit is blended with the C12–C18 fraction from the hydrocracker to form diesel. Since the C12-18 fraction from the Fischer-Tropsch unit will of natural consequence contain oxygenates, alcohols specifically, the blended product will also contain these oxygenates. The text on page 4.3 discloses that the Fischer-Tropsch C12-18 product contains oxygenates.
U.S. Pat. No. 5,506,272 also describes a Fischer-Tropsch diesel fuel containing oxygenates. Table 3 in Column 18 describes a Fischer-Tropsch diesel fuel with a cetane index of 62 and containing 6 wt % alcohols and 6 wt % other oxygenates.
U.S. Pat. No. 6,296,757 discloses a blend of hydrocracked wax with unhydrotreated hot and cold condensates. FIG. 1 illustrates how the product of the invention is a blend of hydrocracked wax and unhydrotreated hot and cold condensates. The unhydrotreated hot and cold condensates contain olefins and oxygenates, and therefore, the product taught in this patent will also contain olefins and oxygenates. In particular Example 2, column 6, lines 26–39 teaches a product (Fuel B). An analysis of Fuel B is shown in Table 1 in column 8. Fuel B contains 0.78 mmol/g of olefins as measured by the Bromine No. and 195 ppm oxygen as oxygenates. This Bromine Number is equivalent to a wt % olefins between 0.7 and 0.98 depending on the assumed molecular weight of the olefins.
U.S. Pat. No. 5,689,031 also discloses a clean distillate useful as a diesel fuel or diesel blending stock produced from Fischer-Tropsch wax made by separating wax into heavier and lighter fractions, further separating the lighter fraction, and hydroisomerizing the heavier fraction and that portion of the light fraction below about 500° F. The isomerized product is blended with the untreated portion of the lighter fraction. FIG. 1 illustrates the process for producing the product as described therein. FIG. 1 illustrates that the product is a blend of hydrocracked wax, hydrotreated cold condensate, and unhydrotreated hot condensate. The unhydrotreated hot condensate contains olefins and oxygenates, and therefore, the product contains olefins and oxygenates. In particular, Example 2, column 6, lines 49–61 teaches a product (Fuel B). An analysis of Fuel B is shown in Table 1 in column 8. Fuel B contains 0.78 mmol/g of olefins as measured by the Bromine No. and 195 ppm oxygen as oxygenates. This Bromine Number is equivalent to a wt % olefins between 0.7 and 0.98 depending on the assumed molecular weight of the olefins.
Similarly, U.S. Pat. No. 5,766,274 discloses a clean distillate useful as a jet fuel or jet blending stock produced from Fischer-Tropsch wax by separating wax into heavier and lighter fractions; further separating the lighter fraction and hydroisomerizing the heavier fraction and that portion of the light fraction above about 475° F. The isomerized product is blended with the untreated portion of the lighter fraction to produce jet fuel.
U.S. Pat. No. 6,274,029 discloses diesel fuels or blending stocks produced from non-shifting Fischer-Tropsch processes by separating the Fischer-Tropsch product into a lighter and heavier fractions, e.g., at about 700° F., subjecting the 700° F.+ fraction to hydro-treating, and combining the 700° F.+ portion of the hydrotreated product with the lighter fraction that has not been hydrotreated.
However, none of these processes as described in the prior art addresses the critical issue of stability of the fuel that is produced. Temperature, time, extent of oxygen exposure, impurities, and fuel composition are all important aspects of fuel stability. Fuel stability is determined by thermal stability and storage stability of the fuel. Thermal stability relates to the stability of the fuel when exposed to temperatures above ambient for relatively short periods of time. Storage stability generally relates to the stability of the fuel when stored at near ambient conditions for longer periods of time. A stable fuel can become unstable due to the introduction of other components, including incompatible fuel components. Components, which can cause a fuel to become unstable, include highly aromatic and heteroatom-rich fuel components, metals, oxidation promoters, and incompatible additives.
ASTM specifications for Diesel Fuel (D985) describe stability measurements for the respective fuels. For diesel fuel, ASTM D6468, “Standard Test Method for High Temperature Stability of Distillate Fuels” is under consideration as a standard test method for a diesel fuel and this test can provide a good measure of the stability of the fuel. Neat Fisher Tropsch products typically have excellent stabilities in this test.
In addition to conventional measurements of stability (thermal and storage), studies by Vardi et al (J. Vardi and B. J. Kraus, “Peroxide Formation in Low Sulfur Automotive Diesel Fuels,” February 1992, SAE Paper 920826) describe how fuels can develop significant levels of peroxide during storage, and how these peroxides can attack fuel system elastomers (O-rings, hoses, etc.). The formation of peroxides can be measured by Infrared spectroscopy, chemical methods, or by the attack on elastomer samples. As described by Vardi et al, fuels can become unstable with respect to peroxide formation when their sulfur content is reduced to low levels by hydroprocessing. Vardi et al also describe how compounds like tetralin can cause fuels to become unstable with respect to peroxide formation, while polycyclic aromatic compounds like naphthalenes can improve stability. Vardi et al. explains that aromatics act as natural antioxidants and notes that natural peroxide inhibitors such as sulfur compounds and polycyclic aromatics can be removed.
Following on the work by Vardi, two recent patents from Exxon describe how the peroxide-stability of highly-paraffinic Fischer-Tropsch products in unacceptable, but can be improved by the addition of sulfur compounds from other blend components. However, since sulfur compounds increase sulfur emissions, this approach is not desirable.
By way of example, U.S. Pat. No. 6,162,956 discloses a Fischer-Tropsch derived distillate fraction blended with either a raw gas field condensate distillate fraction or a mildly hydrotreated condensate fraction to obtain a stable, inhibited distillate fuel. The fuel is described as a blend material useful as a distillate fuel or as a blending component for a distillate fuel comprising: (a) a Fischer-Tropsch derived distillate comprising a C8–−700° F. fraction, and (b) a gas field condensate distillate comprising a C8–−700° F. fraction, wherein the sulfur content of the blend material is ≧1 ppm by wt. This patent discloses that distillate fuels derived from Fischer-Tropsch processes are hydrotreated to eliminate unsaturated materials, e.g., olefins, and most, if not all, oxygenates. This patent further discloses that the products contain less than or equal to 0.5 wt % unsaturates (olefins and aromatics).
Similarly, U.S. Pat. No. 6,180,842 discloses a Fischer-Tropsch derived distillate fraction blended with either a raw virgin condensate fraction or a mildly hydrotreated virgin condensate to obtain a stable inhibited distillate fuel. The fuel is describes as a blend material useful as a distillate fuel or as a blending component for a distillate fuel comprising (a) a Fischer-Tropsch derived distillate comprising a C8–700° F. stream and having a sulfur content of less than 1 ppm by wt, and (b) 1–40 wt % of a virgin distillate comprising a C8-700° F. stream; wherein the sulfur content of the blend material is ≧2 ppm by wt. This patent notes that while there is no standard for the peroxide content of fuels, there is general acceptance that stable fuels have a peroxide number of less than about 5 ppm, preferably less than about 4 ppm, and desirably less than about 1 ppm. This value is tested after storage at 60° C. in an oven for 4 weeks. The patent shows that Fischer-Tropsch products having a peroxide number of 24.06 after 4 weeks have unacceptable stability.
The Fischer-Tropsch products in the '842 patent are described as being >80 wt %, preferably >90 wt %, more preferably >95 wt % paraffins, having an iso/normal ratio of 0.1 to 10, preferably 0.3 to 3.0, more preferably 0.7 to 2.0; sulfur and nitrogen of less than 1 ppm each, preferably less than 0.5, more preferably less than 0.1 ppm each; <0.5 wt % unsaturates (olefins and aromatics), preferably <0.1 wt %; and less than 0.5 wt % oxygen on a water free basis, preferably less than about 0.3 wt % oxygen, more preferably less than 0.1 wt % oxygen and most preferably nil oxygen. The '842 patent teaches that the Fischer-Tropsch distillate is essentially free of acids.
U.S. Pat. No. 5,689,031 demonstrates that olefins in low-sulfur diesel fuel contribute to peroxide formation. See Fuels C and D in Example 7, and FIG. 2. The '031 patent teaches that the solution to the peroxide forming tendency is to limit the olefin content by hydrotreating the lightest olefin fraction. However, this solution requires the use of expensive hydrogen gas.
It is desired to produce a distillate fuel that has low sulfur content economically, preferably without or with minimal expensive hydroprocessing, and obtain a diesel fuel that has acceptable stability, measured in terms of thermal stability, storage stability, and peroxide resistance. Therefore, it is desired that the diesel fuel be able to have a high olefin content, for example greater than or equal to 2 weight %, and exhibit acceptable stability.